1. Field of the Invention
The present invention relates to an image forming apparatus comprising charging means for charging an image bearing body, and developer charge providing means provided in an upstream position of the charging means in the rotating direction of the image bearing body and adapted to charge, in a normal polarity, residual developer remaining on the image bearing body after transfer of a developer image.
2. Related Background Art
In an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus, for the purpose of charging the surface of an image bearing body which is constituted by a body to be charged such as a photosensitive body or a dielectric body, there is conventionally employed a corona charging method which is a non-contact charging method of applying a corona discharge, generated by the application of a high voltage to a thin corona discharge wire, to the surface of the image bearing body-thereby achieving the charging thereof.
on the other hand, in recent years, there is being principally adopted a contact-charging method of contacting a charging member of roller type or blade type with the surface of the image bearing body, constituting the body to be charged, and applying a voltage to the charging member thereby charging the surface of the image bearing body, because of a lower voltage in the process, a lower amount of ozone generation and a lower cost. In particular, the charging member of roller type is capable of stable charging over a prolonged period.
The voltage applied to the charging member can be a DC voltage only, but there may also be applied an oscillating voltage to alternately induce discharge in the positive side and in the negative side, thereby achieving uniform charging.
It is already known, for example, that the application of an oscillating voltage, formed by superposing an AC voltage having a peak-to-peak voltage at least equal to twice of a threshold voltage for starting the discharge on the charged body under the application of a DC voltage (charging start voltage) and a DC voltage (DC offset bias), to the charging member shows an effect of averaging the charge on the charged body, thereby achieving uniform charging. The waveform of the oscillating voltage is not limited to a sinusoidal wave but can also be a rectangular wave, a triangular wave or a pulse wave. The oscillating voltage includes also a voltage of rectangular shape obtained by periodically turning on and off a DC voltage, and a DC voltage of which value is periodically varied to obtain an output same as a superposed voltage of an AC voltage and a DC voltage.
In the following, a contact charging method of effecting charging by applying an oscillating voltage to the charging member will be called “AC charging method”. Also a contact charging method of effecting charging by applying a DC voltage only will be called “DC charging method (system)”.
Recently there is known an image forming apparatus employing a cleanerless system, which is a system for recovering the transfer residual toner, remaining on the photosensitive drum after the image transfer, into a developing apparatus. In the cleanerless system, it is preferable to regulate the electrical polarity of the transfer residual toner to a normal polarity until such transfer residual toner is recovered in the developing apparatus. It is however not easy to regulate the polarity of the transfer residual toner to the normal polarity by the voltage applied by the charging means (regardless whether it is of contact type or non-contact type).
Consequently it is preferred to an auxiliary charging member, constituting developer charge providing means which provides the transfer residual toner with a charge thereby regulating the charge polarity of the transfer residual toner to the normal polarity. The auxiliary charging means is given a predetermined voltage from a power supply apparatus and provides the transfer residual toner, remaining on the photosensitive drum, with a charge. However, since the remaining amount is variable depending for example on the image density, a sufficient charge cannot be provided to the transfer residual toner if the voltage applied to the auxiliary charging member is not appropriate. Also, since the photosensitive drum is charged by the voltage applied from the auxiliary charging member, there may result a situation, depending on the potential after the charging by the auxiliary charging member, where the charging for latent image formation on the photosensitive drum becomes not uniform, thereby deteriorating the image quality.
The contact charging method can resolve the drawbacks such as scraping of the photosensitive drum since the voltage applied to the contact charging means can be made lower in comparison with the case of the non-contact charging method. However, since the contact charging means is in contact with the image bearing body, the toner, the external additive thereof or the like may be deposited on the contact charging member, thereby causing a smear thereon. Such a smear results in an image defect such as a defective charging.
In order to avoid such drawback, an auxiliary charging member such as a brush is provided as the developer charge providing means in the upstream side of the charging member in the rotating direction of the photosensitive drum. Such a method allows to select a low voltage for setting to the charging means. It is also rendered possible to provide the toner or the external additive on the image bearing body with such a charge as not to stick to the charging member, thereby preventing smear of the charging member. Also the contact charging member is rendered capable of stable charging in uniform manner over a prolonged period.